1. Field of the Invention
The present invention relates to an optical receiver and a controlling method thereof, used for an optical transmission system, and more specifically, relates to a control technique for optimizing a dispersion compensation amount and a phase control amount in an optical receiver corresponding to a differential M-phase modulation format.
2. Description of the Related Art
Recently, there are high demands for introduction of a next-generation optical transmission system corresponding to a transmission speed of, for example, 40 Gb/s (gigabits per second), and further, the same transmission distance and frequency efficiency as those of a 10 Gb/s system are required for the optical transmission system. As a means for realizing such a demand, there are active research and development of Return to Zero-Differential Phase Shift Keying (RZ-DPSK) or Carrier-suppressed (CS) RZ-DPSK modulation format, which are excellent in Optical Signal-to-Noise Ratio (OSNR) resistance and nonlinearity resistance, as compared to a Non Return to Zero (NRZ) modulation format heretofore applied to the system of 10 Gb/s or less. Moreover, in addition to the abovementioned modulation format, research and development of a phase modulation format such as Differential Quadrature Phase-Shift Keying (RZ-DQPSK) or CSRZ-DQPSK modulation format having characteristics of high frequency efficiency with narrow spectrum have become active (for example, refer to Japanese Unexamined Patent Publication No. 2003-60580 and Published Japanese translation No. 2004-516743 of PCT International Publication).
FIG. 16 is a block diagram showing a configuration example of a conventional optical transmitter that adopts the 43 Gb/s (CS)RZ-DPSK modulation format to transmit an optical signal. Moreover FIG. 17 is a block diagram showing a configuration example of a conventional optical receiver that performs reception processing such as demodulation with respect to the optical signal transmitted by the optical transmitter in FIG. 16.
The optical transmitter 110 shown in FIG. 16 includes, for example, a transmission data processing circuit 111, a continuous wave (CW) light source 112, a phase modulator 113, and an intensity modulator for RZ pulsing 114.
Specifically, the transmission data processing circuit 111 has a function as a framer that frames input data and a function as a forward error correction (FEC) encoder that applies an error correction code, as well as a function as a DPSK precoder that performs an encoding process, which reflects difference information between a code one bit before and the current code. The phase modulator 113 modulates continuous waves from the CW light source 112 with encoded data from the transmission data processing circuit 111, and outputs an optical signal having constant light intensity, which carries information on a binary optical phase, that is, a DPSK-modulated optical signal. Moreover the intensity modulator for RZ pulsing 114 is for RZ-pulsing the optical signal from the phase modulator 113. In particular, an optical signal RZ-pulsed by using a clock drive signal of the same frequency as the bit rate (43 GHz) and amplitude one times the quenching voltage (Vπ) is referred to as an RZ-DPSK signal. An optical signal RZ-pulsed by using a clock drive signal of a frequency half the bit rate (21.5 GHz) and amplitude two times the quenching voltage (Vπ) is referred to as a CSRZ-DPSK signal. The (CS)RZ-DPSK signal transmitted from the optical transmitter 110 has a 43 GHz clock waveform as the optical intensity, and carries information on the binary optical phase.
Moreover, the optical receiver 120 shown in FIG. 17 is connected to the optical transmitter 110 via a transmission path 101 to receive and process the (CS)RZ-DPSK signal input from the transmission path 101, and includes, for example, a variable dispersion compensator 121, an optical amplifier 122, a delay interferometer 123, a photoelectric conversion circuit 124, a reproducing circuit 125, a received data processing circuit 126, and a control circuit 127. With this optical receiver 120, wavelength dispersion tolerance in 43 Gb/s transmission becomes as strict as about 1/16, as compared to the case of 10 Gb/s transmission. Accordingly, the variable dispersion compensator 121 is arranged at an input end to perform highly accurate wavelength dispersion compensation.
More specifically, the variable dispersion compensator 121 performs wavelength dispersion compensation of the (CS)RZ-DPSK signal transmitted through the transmission path 101. The optical amplifier 122 amplifies the power of the optical signal output from the variable dispersion compensator 121 to a required level in order to compensate light loss in the variable dispersion compensator 121, and outputs the amplified optical signal to the delay interferometer 123. The delay interferometer 123 comprises for example, a Mach-Zehnder interferometer, and makes a delay component of one bit time (in this case, 23.3 ps) of an input signal and a component phase controlled with 0 rad interfere with each other (delay interference), and outputs an interference result as two outputs. In other words, one of branching waveguides constituting the Mach-Zehnder interferometer is formed so as to be longer than the other branching waveguide by a propagating length corresponding to one bit time. The photoelectric conversion circuit 124 comprises a dual-pin photodiode that performs balanced detection by receiving the two outputs from the delay interferometer 123, respectively. The reproducing circuit 125 is for extracting a data signal and a clock signal from the received signal, which has been subjected to balanced detection in the photoelectric conversion circuit 124. The received data processing circuit 126 executes signal processing such as error correction based on the data signal and the clock signal extracted by the reproducing circuit 125. The control circuit 127 monitors the number of occurrences of errors detected at the time of error correction processing in the received data processing circuit 126, and feed-back controls the variable dispersion compensator 121 and the delay interferometer 123 so that the number of occurrences of errors becomes the least.
As the conventional technique associated with the control of the variable dispersion compensator and the like in the optical transmission system applying the optical modulation format such as (CS)RZ-DPSK described above, a technique in which the quality of the received optical signal is monitored without performing the demodulation process of the received optical signal has been proposed in, for example, U.S. Patent Application Publication No. 2004-0223769.
Moreover, a technique in which a variable dispersion compensator and the like provided in a transmission section, a relay or a reception section is feed-back controlled and optimized based on a transmission characteristic measured at a receiving end is disclosed in, for example, Japanese Unexamined Patent Publication No. 8-321805 and Japanese Unexamined Patent Publication No. 2000-115077.
As described above, in the conventional optical receiver, in order to receive and process the optical signal having a super high-speed bit rate as high as for example 40 Gb/s and adopting the (CS)RZ-D(Q)PSK modulation format, not only the phase control amount in the delay interferometer but also the dispersion compensation amount in the variable dispersion compensator need to be optimized and controlled according to the monitored number of occurrences of errors in the demodulated electric signal. However, the characteristic of the wavelength dispersion compensation amount and the characteristic of the phase control amount relative to the number of occurrences of errors in the received signal are different in nature. Therefore, at the time of initial setup, since the control amounts of the variable dispersion compensator and the delay interferometer deviate from the optimum value, the optimum control amount for the both devices needs to be searched. However, the search requires a relatively long time, thereby causing a problem in quickly stabilizing the control amount of the delay interferometer and the variable dispersion compensator.
FIG. 18 shows one example of a situation in which the optimum control amount of the variable dispersion compensator and the delay interferometer in the conventional optical receiver is searched. In this example, the optimum control amount for both devices indicated in a circular area enclosed by the broken line in the figure is searched by alternately adjusting the dispersion compensation amount in the variable dispersion compensator and the optical phase control amount in the delay interferometer. Moreover, FIG. 19 shows one example of a relation between the dispersion compensation amount in the variable dispersion compensator, and the optical phase control amount in the delay interferometer, and the number of occurrences of errors in the received signal. Thus, since an optimum point at which the number of occurrences of errors in the received signal becomes the fewest changes depending on the dispersion compensation amount and the optical phase control amount, respectively, the optimum point for each of the dispersion compensation amount and the optical phase control amount needs to be searched. When each search is performed using the received signal with excellent OSNR at the time of normal operation, then as shown in FIG. 20, a long time is required for counting the number of occurrences of errors corresponding to the required error rate (for example, in the case of a received signal of 40 Gb/s, 25 seconds are required for counting errors of 10−12 level). An optimum point P′ searched in such a state that the errors cannot be counted sufficiently may be different from an original optimum point P.